It's really been tough on everyone in this country watching the story unfold in boston these last two weeks. But when terror like this strikes, many of us ask, what would we do if disaster struck in tonight's "20/20" examines what to do in an instant, to save your family. Tonight here, abc's matt gutman on why basic instincts aren't always the best ones. Reporter: There is an instant, when chaos erupts. That humans decide to join a fleeing herd or become heroes. That instant is really amazing. Some of us are hardwired to run. And other people are hardwired to fight. Reporter: In boston, dozens chose not to run. But just this weekend in denver -- 20,000 people at a rally stampeded. So, what makes most people act the way they do? What's fascinating about crowds is that we all tend to follow them. Reporter: Just look at a classic psychological experiment ill stated here. A woman sits quietly doing her work smoke starts pouring in. She's visibly worried. Still, five minutes go by. The other people in the room, all in on the experiment, ignore the smoke. She's just sitting there. No one else is reacting. Reporter:10, 15, 20 more minutes pass. She still doesn't move because the others don't. That's how powerfully socialized we are. Reporter: Socialized to death in some cases. But psychologists say some people are hardwired to run against the grain. Surging towards the flash point. When I stepped into the street, you get a sense of tunnel vision. I ran as fast as I could to the epicenter of the blast. Reporter: Michael chase is a few pete away from dzhokhar cz tsarnaev. After the blast, he ran to this little girl to help. It's just something you do. Reporter: A hardwired hero. Matt gutman, abc news, miami. We will continue to myth

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